1. Field of the Invention
The invention relates to minimizing dynamic structural loads of an aircraft, introduced by an external excitation to the same.
2. Background Information
Dynamic structural loads are introduced to the structure of an aircraft by external excitations, e.g. due to wind, gusts, turbulences or similar influences, and by excitations due to pilot/flight control system demands. As a result of the flexible properties of the aircraft structure, such excitation may result in oscillations or vibrations that may exceed a given magnitude, so that they can be harmful to the aircraft structure, especially of frequencies in the range of natural or characteristic oscillations of the flexible aircraft structure. If it is desired to reduce the mass of the aircraft structure at high aspect ratio of wing and fuselage, it is necessary to take measures against excessive dynamic structural loads.
Atmospheric gusts excite dynamic wing loads especially in the wing roots as well as vertical accelerations critical for passenger safety. Besides manoeuvre loads these loads are sizing for the aircraft structure. If it is possible to reduce said loads, then a lighter wing design is possible, improving fuel efficiency and thus reducing direct operating costs. Another advantage is the increase of passenger safety due to alleviation of vertical accelerations.
Document DE 198 41 632 C2 discloses a method for compensating structural oscillations of an aircraft, which are introduced by an external excitation, comprising detecting at least one body rate of the aircraft with a sensor arrangement, providing said at least one body rate to a flight controller, and producing movements of control surfaces of the aircraft to minimize the excited oscillations.
For updraft gust the first load peak can be eliminated by a rapid deflection of spoilers on the upper wing surface, see Airbus patent US 20080265104 A. This approach is limited by control authority (reaction time, actuator power), by the loads that the rapid spoiler deflection introduces to the wing structure, as well as by the constraint that the second peak of the wing bending acceleration must not be increased by the spoiler deflection.
EP 1 814 006 A1 proposes the symmetric actuation of ailerons for gust load alleviation using a high pass filtered alpha probe signal as reference for the gust and an adaptive feed-forward controller. This approach solves the problems mentioned above, but is even more limited by the control authority of the ailerons.
Further apparatuses and methods for reducing vibrations due to gust are disclosed in DE 10 2008 014 236 A1 and in EP 1 854 717 A1. In EP 1 854 717 A1, there is disclosed how control commands for alleviating disturbances can be generated. DE 10 2008 014 236 A1 describes an apparatus and a method for adaptive control.